1. Field of the Invention
The present invention relates to a contactless power transmission circuit for transmitting power in a contactless manner to a power receiving device placed on a placing portion.
2. Description of the Related Art
In recent years, there has been known a contactless power transmission circuit provided with a resonant circuit including a power transmission coil for transmitting power to a power receiving device and a resonance coil which resonates with this power transmission coil, a switching elements for resonating this resonant circuit and a driving circuit for driving this switching element, and adapted to transmit power in a contactless manner to the power receiving device placed on a placing portion and charge the power receiving device (see, for example, patent literature 1 (Publication of JP No. 3416863), patent literature 2 (Publication of JP No. 3363341)).
In such a contactless power transmission circuit, the inductance of the power transmission coil is smaller when the power receiving device is not placed on the placing portion than when it is placed. Thus, a current flowing into the power transmission coil increases, resulting in a problem of increasing standby power.
However, patent literature 1 aims to stably oscillate the resonant circuit and patent literature 2 aims to miniaturize the circuit. Neither of the literatures aims to reduce the standby power.
Accordingly, patent literature 3 (JP-A No. 2008-236916) discloses a contactless power transmission circuit provided with a power transmitting device including a primary coil and a power receiving device including a secondary coil, wherein a change in the self-inductance of the primary coil is detected immediately after a power feeder starts feeding power to the primary coil, and the power feeding of the power feeder is stopped if a detected value of this change is below a specified value while being continued if the detected value is equal to or above the specified value.
Further, patent literature 4 (JP-A No. 2000-166129) discloses technology enabling a reduction of standby power by detecting an input current input to an electromagnetic transformer from a commercial power supply via a rectifying circuit and causing a switching element to intermittently operate by a controller when this input current is equal to or below a set value to suppress power consumption of a charger during non-charging periods.
However, a series resonant circuit is built by the primary coil and a capacitor in patent literature 3. Since this series resonant circuit is not resonated by turning the switching element on and off, the principle of oscillation is totally different from the invention of this application.
Further, in patent literature 4, the switching element is connected to a primary side power feeding coil. Since this switching element does not oscillate the primary side power feeding coil, the principle of oscillation is totally different from the invention of this application. Further, in patent literature 4, charging and non-charging are detected by detecting the input current input to the electromagnetic transformer from the commercial power supply via the rectifying circuit. Since the input current is unstable by being influenced by the commercial power supply, there is a problem of being unable to accurately detect charging and non-charging.